Day: January 12, 2009

This is the dingus that attaches the crossbeam to the central pipe rising up from the table for my counterweight gantry. I discarded a whole bunch of elaborate construction ideas in favor of just jamming a plug in the pipe and cranking down on a nut to tighten it.

It’s pretty much self-explanatory; I cut everything to fit, cleaned up the cuts with a file, and added some lube to the tapers so as to make it nice & slippery.

The need for an O-ring to hold the halves together occurred to me after I’d bandsawed a 1 mm trench in the side of the plug. I chucked it up in the lathe again and used a round-nose tool to carve a groove around its belly. If you try this, do the groove first: an interrupted cut is murder on what’s basically a parting-off tool.

While I know (thanks to Guy Lautard’s invaluable Machinist’s Bedside Reader books) that a self-releasing plug must have a taper angle with a tangent greater than the joint’s coefficient of friction, that really wasn’t much help here. I picked 40 degrees and, yup, it’s self-releasing, but not really slippery enough. Takes a bit of torque to expand the plug enough for a good grip.

Perhaps my grubby surface finish has something to do with it?

Memo to self: find out how to figure the taper angle correctly, then do better finishing.